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According to the tech specs, there's a combined digital/audio line in.

https://support.apple.com/kb/SP505?locale=en_US

I have the late 2009 Mini and have recorded from the line in without a problem. (Well, audio line is is prone to hum, but that's usually not the computer's fault.)

I have used my early 2009 Mac Mini to digitise material I have on tape using Garage Band and iTunes. Yes, there was some hum from the line in when I first tried. As I recall the first one I tried was supposedly high quality, with gold plated plugs. I experimented a different cables and eventually found one that works fine.
 
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